二叉树的路径和

/**
* Definition of TreeNode:
* public class TreeNode {
*     public int val;
*     public TreeNode left, right;
*     public TreeNode(int val) {
*         this.val = val;
*         this.left = this.right = null;
*     }
* }
*/
public class Solution {
    /*
     * @param root: the root of binary tree
     * @param target: An integer
     * @return: all valid paths
     */
    private int sum = 0;
    private List<List<Integer>> result = new ArrayList<List<Integer>>();
    private List<Integer> path = new ArrayList<Integer>();
    public List<List<Integer>> binaryTreePathSum(TreeNode root, int target) {
        // write your code here
        calculate(root, target);
        return result;
    }
    
    public void calculate(TreeNode node, int target) {
        if(node != null) {
            sum += node.val;
            path.add(node.val);
            if (sum == target && node.left == null && node.right == null) {
                List<Integer> temp = new ArrayList<Integer>(path);   //拷贝path
                result.add(temp);
                sum -= node.val;                //回溯到父节点状态
                path.remove(path.size()-1);     //回溯到父节点状态
                return;
            }
            if (node.left == null && node.right == null) {
                sum -= node.val;               //回溯到父节点状态
                path.remove(path.size()-1);    //回溯到父节点状态
                return;
            }
            calculate(node.left, target);
            calculate(node.right, target);
            sum -= node.val;                   //回溯到父节点状态
            path.remove(path.size()-1);        //回溯到父节点状态
        }
    }
}

代码有点乱，有待改进。